docs: Validate JSON snippets (settings, keymap, tasks, etc) (#40043)

Closes #ISSUE

Release Notes:

- N/A *or* Added/Fixed/Improved ...

docs/src/extensions/icon-themes.md

@@ -17,7 +17,7 @@ Each icon theme file should adhere to the JSON schema specified at [`https://zed
 
 Here is an example of the structure of an icon theme:
 
-```json
+```json [icon-theme]
 {
   "$schema": "https://zed.dev/schema/icon_themes/v0.3.0.json",
   "name": "My Icon Theme",
@@ -34,8 +34,8 @@ Here is an example of the structure of an icon theme:
         "stylesheets": {
           "collapsed": "./icons/folder-stylesheets.svg",
           "expanded": "./icons/folder-stylesheets-open.svg"
-        },
-      }
+        }
+      },
       "chevron_icons": {
         "collapsed": "./icons/chevron-right.svg",
         "expanded": "./icons/chevron-down.svg"